Due to close similarity in crystal structure to quartz, AlPO.sub.4 has attracted considerable attention recently as a new piezoelectric material for traditional resonator applications. Quartz has become the most important technological piezoelectric material by virtue of the existence of several quartz crystal orientations that produce zero dependence of frequency on temperature. Because of the exceedingly close resemblance in crystal structure between quartz and AlPO.sub.4, similar kinds of zero temperature coefficient orientations are predicted for crystals of AlPO.sub.4. Another reason for the interest in AlPO.sub.4 is the high coupling efficiency obtainable with this material. The coupling coefficients for AlPO.sub.4 are approximately three times those of quartz.
The predominant AlPO.sub.4 crystal structure is patterned directly after the quartz structure with Al and P atoms replacing the Si atoms. The striking similarity between the two crystals can be attributed to the small size difference between Si and Al or P. However, the elastic constants are quite different and the temperature dependence of the elastic constants are not well developed. Information regarding the elastic properties of various AlPO.sub.4 crystal cuts is given in IEEE Transactions on Sonics and Ultrasonics, Vol. SU-23, No. 2, March 1976, pp. 127-135, Journal of Applied Physics, Vol. 48, No. 3, March 1977, pp. 887-892.
AlPO.sub.4 is synthesized by hydrothermal techniques. Conventional hydrothermal approaches have been unsuccessful so far in reproducibly growing crystals having the size and quality of hydrothermally grown quartz. However, following extensive efforts by us, useful piezoelectric crystals of AlPO.sub.4 were prepared hydrothermally using phosphoric acid as the hydrothermal solvent. See E. D. Kolb and R. A. Laudise, Journal of Crystal Growth, 43, (1978) pp. 313-319. This effort followed earlier ones of others, along similar lines, using the same growth solution (see, e.g., J. M. Stanley, Industrial and Engineering Chemistry, 46, (1954) page 1684, and W. von Jahn and E. Kordes, Chem. Erde, (1953) page 75).
Because of the limited success of prior efforts, the need exists for alternative procedures for preparing crystals of aluminum orthophosphate that offer potential for achieving the goal of large, high quality crystals comparable to routinely synthesized quartz crystals.